The long term goal of this project is to define the mechanism(s) of bovine leukosis virus (BLV) pathogenesis in cattle as an animal model for human retrovirus-associated lymphoid malignancies. Our working hypothesis is that BLV-infected T lymphocytes are pivotal to the abnormal lymphoproliferation. This hypothesis will be tested by extensive characterization of the tropism of BLV for mononuclear leukocyte subpopulations and associated molecular events that lead to lymphocytosis and/or tumor development. Cell subpopulations from all major lymphoid compartments will be analyzed from animals at multiple stages of disease progression (aleukemic, lymphocytotic & tumor-bearing stages). Mononuclear leukocyte subpopulation perturbations will be identified by flow cytometry and immunohistology. Viral tropism for mononuclear subpopulations will be determined by identification of cells harboring provirus (Southern hybridization & PCR). The ability of such cells to express BLV proteins will be determined by immunofluorescence staining of intracellular antigen (flow cytometry & immunohistology); evidence of cellular expression of BLV proteins with potential cell-growth regulatory activity (specifically the trans-activating protein, tax) will be by Northern hybridization- and/or PCR-based identification of mRNA. The genetic clonality of T and B. cells will be by identification of Ig and TCR gene rearrangement. A role for BLV tax-driven T lymphocyte production of B and/or T cell growth factor(s) will be determined by functional assays as well as Northern hybridization- and/or PCR-based identification of interleukin mRNA. Association of all data obtained, with the state of disease progression, will greatly increase our understanding of BLV pathogenesis and provide additional insight, at the cellular and molecular level, into retrovirus-mediated dysregulation of lymphocyte proliferation.